For purposes of this disclosure, a bellows is defined as any tube-like conveyances or compliant cavities with substantial compliance along the axis of motion, which provide a boundary over which a pressure difference can be maintained and which are intended in operation to undergo axial compression, as a desired part of their function. As such, this definition includes devices such as described in U.S. Pat. Nos. 6,237,922 and 6,755,027, the entire contents of both of which are incorporated herein by reference. This definition also include other devices not specifically named as bellows, as would be recognized by a person of skill in the art. “Conventional bellows” will be used to refer to bellows with essentially uniform stiffness properties along the axis of the bellows structure, as is well approximated by a number of identical repeated structural elements along the compression axis of the bellows. When referring to “formed bellows”, aspects relevant to the sort of corrugated structures described by the Standards of the Expansion Joint Manufacturers are described. Bellows may be formed from a variety of materials, including metal and elastomeric material, and may be formed in a variety of ways.
The field of use of bellows seals on reciprocating pistons is quite old and well developed. In reciprocating machine applications, wherein part of the expected operation of the bellows is that it undergo axial compression and yet act as a piston seal at relatively high frequencies, problems associated with the relatively slow wave speed in bellows materials are a recognized issue, and various methods have been devised to compensate. (For a discussion of the problems, and methods to compensate see, for example. U.S. Pat. Nos. 3,875,806 to Brewster and U.S. Pat. No. 4,556,369 to Braun, wherein external devices are employed to stabilize the bellows and multiple attachments are made to the bellows, intermediate between the ends.)
The recognized problem can be summarized in this manner: bellows are designed to be compliant structures, and yet they have non-trivial mass, so that the propagation rate of a disturbance initiated at one end, traveling through the bellows is much slower than in the bulk media of which the bellows are constructed. As such, standing waves form and this causes stress distributions that are locally higher and different in distribution from what is observed in the static compression or extension case. In general, for applications in reciprocating machinery, it is desirable to ensure that the stresses in the bellows are minimized so that the part will have the greatest fatigue resistance, and thus a large lifetime.